1. Field of the Invention
This invention relates to an exhaust-gas recirculation system for the internal combustion engine that reduces the formation of pollutants in the exhaust gas by lowering the combustion temperature by recycling part of the exhaust gas back to the intake system of the engine.
2. Description of the Prior Art
In internal combustion engines, especially gasoline engines, large quantities of nitrogen oxide (NO.sub.x) is formed as the combustion temperature rises under certain operating conditions. To prevent the formation of NO.sub.x, an exhaust-gas recirculation system (hereinafter abbreviated EGR system) that lowers the combustion temperature by sending a portion of the inert exhaust gas back to the combustion chamber has been used. The EGR system usually opens or closes an exhaust-gas recirculation (EGR) valve that controls the amount of the returned exhaust gas according to operating conditions that manifest themselves as changes in the cooling water temperature and engine vacuum. There is an EGR system, for example, that introduces the engine vacuum from the intake system through a change-over valve to a diaphragm chamber to open or close and exhaust-gas recirculation (EGR) valve connected to the diaphragm at a preset time. With another EGR system, operating conditions of the engine are translated into electric signals. By using such signals, the amount of EGR valve opening needed to achieve the desired operating condition is calculated. Then, the control device outputs such a signal that allows the EGR valve to open or close to the extent desired. With the latter type, a desired amount of the valve opening a is set based on the the operating conditions of the engine at sampling intervals of Ts as shown in FIG. 1. As the desired amount a varies and deviates by .epsilon..sub.1 from the actual amount of lift b, which represents the extent to which the EGR valve actually opens or closes, the control device generates a pulse having a time-width T.sub.pwl that is proportional to a difference .epsilon. .sub.1, which is then translated into an actuating signal f for a valve opening solenoid. After this, a quiescent time T.sub.w of a given length is given to stabilize the control. Once the pulse T.sub.pw1 is outputted, however, it is impossible to proceed to the next action until the quiescent time T.sub.w is over. Thus, even if the desired value a changes at point T.sub.2, the actuating signal f continues the output with a time-width T.sub.pw1. It is not until point T.sub.3 is reached after the passage of the quiescent time T.sub.w that a pulse having a time-width T.sub.pw2 proportional to a difference .epsilon..sub.2 is outputted. As will be understood from the above, the conventional EGR system of the type being described cannot avoid wasting as much time as T.sub.L. Accordingly, the actual position (or amount of lift) b of the EGR valve can be adjusted in close conformity with such a gentle change in the desired value a as shown in FIG. 2(a). But close follow-up sometimes becomes impossible when changes are as sudden as those shown at (b) and (c) in FIG. 2. In FIGS. 1 and 2, reference character f designates an actuating signal that causes the EGR valve to move in the opening direction and reference character g denotes an actuating signal that causes the same valve to move in the closing direction.